But it will signify more than that. The near-elimination of Scottish Labour would mark a generational shift, echoing previous transfers of the popular will from the Liberals to the Unionists and the Unionists to Labour. We will not merely be changing political parties but confirming beyond any doubt that Scotland has changed.

Nowhere is that more vividly symbolised than in Coatbridge, Chryston and Bellshill, not only one of Labour’s safest seats in Scotland but one of their safest anywhere in the UK. To find the last time Coatbridge, Chryston and Bellshill and its predecessor constituencies voted for a party other than Labour you have to go back to 1931, when the Unionists narrowly defeated miner and writer James C Welsh. The incumbent MP Tom Clarke won in 2010 with 67% of the vote and a majority of more than 20,000; only once, in 1983 and facing an SDP challenge, has Clarke taken less than 55% of the vote.

According to Lord Ashcroft, the Nationalists are three points ahead in Coatbridge.

Coatbridge.

The Brig.

Little Ireland.

Leningrad-in-Lanarkshire.

Voting SNP.

Six months ago, I would sooner have believed that Rangers was preparing to sign the Pope as their new striker. Now, I’m not so sure. The term “safe Labour seat” has become an oxymoron. There are no safe Labour seats, only SNP seats temporarily in Labour hands.

For this trend is replicated across the 13 other Labour-held constituencies polled by Lord Ashcroft’s researchers and it points to a cataclysmic result for an outfit once deemed Scotland’s Party. This tectonic shift cannot be explained by something transitory, like opposition to the Conservative-Liberal Democrat coalition’s austerity policies. Those surveyed were, on the whole, optimistic about the economic future both of the country and of their own family.

This is not about economics, primarily. It is about identity and a wholesale redrawing of political allegiances in Scotland.

While this is a terrible poll for Scottish Labour, it may also be their last best chance.

Labour’s age-old line goes: If you vote SNP, you get the Tories. They’ve even put a video out today with that very message:

What this poll illustrates, in dramatic fashion, is that if you vote SNP, you get the SNP. But what then?

It is Scottish Labour’s task to turn voters’ attention to the exquisitely boring but decisive business of parliamentary arithmetic. Because handing the SNP most or all of Scottish Labour’s 40 seats is to hand Ed Miliband a 40-seat handicap in trying to form a Labour government. That may be a good thing. Miliband is far from prime ministerial and Labour has taken Scotland for granted one too many times. Maybe another term for David Cameron and the Conservatives is a price worth paying to punish Labour and install a new dominant party.

But it is a price and honesty requires that the Nationalists be up-front about it. It’s not true, as Scottish Labour is claiming, that David Cameron is eager for big SNP gains on May 7. The Prime Minister is an instinctive Unionist and would rather see the Cotswolds fall to ISIS than deal with a phalanx of pesky separatists demanding more constitutional concessions and another referendum. But if Scotland turning yellow leaves Labour trailing the Tories in number of seats, and Labour refuses to cut a deal with the SNP, then Mr Cameron’s path back to Downing Street, perhaps at the head of a minority government, becomes much easier.

It is unthinkable that the SNP would strike even a confidence and supply deal with the Conservatives, though the poll shows 41% of Nationalist voters are satisfied with Mr Cameron or prefer him to Ed Miliband, against 38% who would rather see the Labour leader in Number 10. It is, however, far from guaranteed that Labour will want to do a deal with an SNP that has just slaughtered their comrades north of the border. They’ll be bruised, resentful, and in no mood for negotiations.

There are much more hard-headed factors at work too. Labour, if it ends up with fewer seats than the Tories in England, cannot afford to be seen as an “illegitimate” government foisted upon a hostile public by an unholy alliance with Scottish Nationalists. Then there is the re-emerging rift between Labour leftists and modernisers, particularly on the role of the private sector in the NHS. To cut a deal with the SNP, which insists it will vote to “protect” the NHS (that is, prevent increased involvement of the private sector), is to invite a re-run of the Blairite-vs-Brownite cold war of the last two decades.

What does Scottish Labour have in its favour? A confident leader in Jim Murphy, a steel-tough street-fighter in Margaret Curran, and a strategic brain in Douglas Alexander. But will it be enough this time?

There is, of course, the possibility that Unionists will vote tactically to keep the SNP out. The Spectator editor and influential Conservative commentator Fraser Nelson writes:

“I’d like the Tories to win the next election, but not as much as I want Jim Murphy to do well. If Ashcroft’s poll is right, then the end of Britain is once again on the cards. The collapse of Scottish Labour will have brought a new constitutional crisis to England’s door and it will be harder than ever to talk about ‘British politics’… Labour would be unwise to expect a dead cat bounce – the Scottish Tories have been waiting 18 years for theirs.

“I’d rather spend a lifetime in a Labour-run Britain than a day in a fractured, diminished, disunited kingdom – and this is what this election now threatens. We thought the union had been saved (just) in the referendum. But the collapse of the last powerful unionist party in Scotland suggests that the battle for Britain may have only just begun.”

Tactical voting is fraught with risk. In some places, it might prevent a Nationalist victory but in others it would make little difference. It would also hand the SNP another battering ram against Labour: They’re back in alliance with the Tories. They just haven’t learned, have they?

Amidst all the calculations about what this poll and its realisation would mean for Scottish Labour, we shouldn’t forget that Scots aren’t just abandoning their old party; they’re embracing a new one. The SNP has done the hard work over many long years to claw Glasgow and the west away from Labour. If they are successful in May, only the most churlish Labour diehard could begrudge them their victory. That victory, however, comes with strings attached.

To be effective powerbrokers at Westminster, the SNP will have to cut deals that will displease some of its supporters and cast votes that will anger others. It will have to become a political party again, not just a movement. That party will have splits, disputes, leaks, and internal wrangling. Veteran Nationalists understand this but the party has to prepare its younger and more excitable new members for the messy compromises and prosaic realities of UK-wide politics. If the SNP can pull off the balancing act required to succeed at Westminster while retaining the enthusiasm and idealism of its supporters, it will be well placed to replicate Labour’s many decades as the national party of Scotland.

That is what the SNP stands to become and the status Labour is set to lose, according to today’s and other polls. Scotland’s national party; the only game in town. Labour would be banished to the political fringes. Coatbridge no more. Airdrie no more. Paisley no more. Motherwell no more. Glasgow no more. We are in the middle of history being made. As for Labour, its MPs and activists could be forgiven for reflecting moodily: “O judgment! thou art fled to brutish beasts/ And men have lost their reason.” But the problem is not the voters; it is Labour and its toxic brew of arrogance, complacency, and mediocrity.

Labour needs to realise this and act on it. Apologise to Scotland for letting it down, pledge that a Labour government will launch a constitutional convention to deliver a federal UK, then hammer the SNP relentlessly on its failings at Holyrood. Labour did this effectively on Tuesday with an attack on the Scottish Government’s record on A&E waiting times. Do it every single day from now until May 7 and Labour could close the gap enough to stave off a complete wipeout.

Three months to go until polling day and we are five minutes to midnight in the Labour era in Scotland. If the party doesn’t launch a powerful fightback, and launch it now, it could be a very long time before it sees dawn again.